Heart failure affects 10% of North Americans and is the leading hospital discharge diagnosis. The diagnosis of heart failure is accompanied by a survival outlook that is comparable to a major cancer. There are limited rehabilitation options available to patients who are suffering with heart failure, and few strategies actually rehabilitate the heart. Cardiac transplantation remains the gold-standard therapeutic intervention for patients with end-stage heart failure, with an increasing number of individuals being added to the transplant waiting list every year. However, wider application of this life-preserving intervention is limited by the availability of donors. Data from the International Society of Heart and Lung Transplantation Registry shows that cardiac transplantation is in progressive decline in suitable donors (2007, Overall Heart and Adult Heart Transplantation Statistics). Two hundred and fifty eight Canadians have died during the last decade (2000-2010; Heart and Stroke Foundation of Canada) while waiting for heart transplantation. Similarly, in the United States, 304 patients died in 2010 alone while waiting for heart transplantation (Organ Procurement and Transplantation Network, U.S. Dept. of Health & Human Services). This phenomenon is primarily due to a shortage of suitable organ donors, and it is being experienced across the globe.
Time is of the essence for removal of a heart from a donor and its successful transplantation into a recipient. The following conventional principles generally apply for optimal donor heart preservation for the period of time between removal from the donor and transplantation: (i) minimization of cell swelling and edema, (ii) prevention of intracellular acidosis, (iii) prevention of injury caused by oxygen free radicals, and (iv) provision of substrate for regeneration of high-energy phosphate compounds, particularly adenosine triphosphate (ATP), during reperfusion. There are two main sources of donor hearts for transplantation. First, breathing patients who have suffered irreversible loss of brain function as a result of blunt head trauma or intracerebral hemorrhage. Such a patient is referred to as a “brainstem-dead” donor or a donor after brain death (“DBD”). Second, patients who have suffered circulatory death. Such a patient is referred to as a “non-heart-beating” donor, a “cardiac dead” donor, a donor after cardiac death, or a donor after circulatory death (DCD).
Brainstem-dead donors can be maintained under artificial respiration for extended periods of time to provide hemodynamic stability throughout their bodies until the point of organ retrieval. Cardiac perfusion is uncompromised and organ functionality is theoretically maintained. However, brainstem death itself can profoundly affect cardiac function. The humoral response to brainstem death is characterized by a marked rise in circulating catecholamines. Physiological responses to this “catecholamine storm” include vasoconstriction, hypertension and tachycardia, all of which increase myocardial oxygen demand. Increased levels of catecholamine circulating throughout the vascular system induce vasoconstriction, which, in turn, compromises myocardial oxygen supply and can lead to subendocardial ischemia. This imbalance between myocardial oxygen supply and demand is one factor implicated in the impairment of cardiac function following brainstem death (Halejcio-Delophont et al., 1998, Increase in myocardial interstitial adenosine and net lactate production in brain-dead pigs: an in vivo microdialysis study. Transplantation 66(10):1278-1284; Halejcio-Delophont et al., 1998, Consequences of brain death on coronary blood flow and myocardial metabolism. Transplant Proc. 30(6):2840-2841. Structural myocardial damage occurring after brainstem death is characterized by myocytolysis, contraction band necrosis, sub-endocardial hemorrhage, edema and interstitial mononuclear cell infiltration (Baroldi et al., 1997, Type and extent of myocardial injury related to brain damage and its significance in heart transplantation: a morphometric study. J. Heart Lung Transplant 16(10):9941000). In spite of no direct cardiac insult, brainstem-dead donors often exhibit reduced cardiac function, and the current understanding is that only 40% of hearts can be recovered from this donor population for transplantation.
Numerous perfusion apparatus, systems and methods have been developed for ex vivo maintenance and transportation of harvested organs. Most employ hypothermic conditions to reduce organ metabolism, lower organ energy requirements, delay the depletion of high energy phosphate reserves, delay the accumulation of lactic acid, and retard morphological and functional deteriorations associated with disruption of oxygenated blood supply. Harvested organs are generally perfused in these systems with solutions comprising antioxidants and pyruvate under low temperatures to maintain their physiological functionality.
The short-comings of hypothermic apparatus, systems and methods have been recognized by those skilled in these arts, and alternative apparatus, systems and methods have been developed for preservation and maintenance of harvested organs at temperatures in the range of about 25° C. to about 35° C. (this can be referred to as “normothermic” temperatures, though normothermic more conventionally means a normal body temperature, i.e., an average of about 37° C.). Normothermic systems typically use perfusates based on the Viaspan™ formulation (also known as the University of Wisconsin solution or UW solution) supplemented with one or more of the following: serum albumin as a source of protein and colloid; trace elements to potentiate viability and cellular function; pyruvate and adenosine for oxidative phosphorylation support; transferrin as an attachment factor; insulin and sugars for metabolic support; glutathione to scavenge toxic free radicals as well as a source of impermeant; cyclodextrin as a source of impermeant, scavenger, and potentiator of cell attachment and growth factors; a high Mg2+ concentration for microvessel metabolism support; mucopolysaccharides for growth factor potentiation and hemostasis; and endothelial growth factors. For instance, Viaspan comprises potassium lactobionate, KH2PO4, MgSO4, raffinose, adenosine, glutathione, allopurinol, and hydroxyethyl starch. Other normothermic perfusion solutions have been developed and used (Muhlbacher et al., 1999, Preservation solutions for transplantation. Transplant Proc. 31(5):2069-2070). While harvested kidneys and livers can be maintained beyond twelve hours in normothermic systems, normothermic bathing and maintenance of harvested hearts by perfusion beyond 12 hours results in deterioration and irreversible debilitation of the hearts' functionality. Another disadvantage of using normothermic, continuous-pulsed-perfusion systems for maintenance of harvested hearts is the time required to excise a heart from a donor, mount it into the normothermic perfusion system and then initiate and stabilize the perfusion process.
After the excised donor heart has been stabilized, its physiological functionality is determined and, if transplantation criteria are met, the excised heart is transported as quickly as possible to a transplant facility.
In the case of brainstem-dead donors, the heart generally is warm and beating when it is procured. It is then stopped, cooled, and put on ice until it is transplanted. Chilling the harvested heart reduces its metabolic activity and related demands by about 95%. However, some metabolic activity continues with the consequence that the heart muscle begins to die, and clinical data have shown that once the period of chilling of a harvested heart is prolonged beyond 4 hours, the risk of 1-year mortality post-transplant starts to rise. For example, risk of death at 1-year post-transplant for a recipient receiving a heart that has been preserved by chilling for six hours more than doubles compared to a recipient receiving a heart that has been chilled for less than 1 hour (Taylor et al., 2009, Registry of the International Society for Heart and Lung Transplantation: Twenty-sixth Official Adult Heart Transplant Report—2009. JHLT 28(10):1007-1022).
Well-defined criteria have been developed for harvesting organs for transplantation from non-heart-beating donors (Kootstra et al., 1995, Categories of non-heart-beating donors. Transplant Proc. 27(5):2893-2894; Bos, 2005, Ethical and legal issues in non-heart-beating organ donation. Transplantation, 2005. 79(9): p. 1143-1147). Non-heart-beating donors have minimal brain function but do not meet the criteria for brainstem death, and therefore such donors cannot be legally declared brainstem dead. When it is clear that there is no hope for meaningful recovery of the patient, the physicians and family must be in agreement to withdraw supportive measures. Up to this point in care, non-heart-beating patients are often supported with mechanical ventilation as well as intravenous inotropic or vasopressor medication. However, only those patients with single system organ failure, namely failure of the neurologic system, can be considered for organ donation. Withdrawal of life support, most commonly the cessation of mechanical ventilation, is followed by anoxic cardiac arrest, after which the patient must remain asystolic for five minutes before organ procurement is allowed. Consequently, the organs of non-heart-beating donors are necessarily exposed to variable periods of warm ischemia after cardiac arrest, which may result in varying degrees of organ damage. However, provided that the duration of warm ischemia is not excessive, many types of organs, such as kidneys, livers, and lungs, can be harvested from non-heart-beating donors and are able to recover function after transplantation with success rates that approximate those for transplanted organs from brainstem-dead donors. While hearts harvested from brain-dead donors are exposed to an ischemic period limited to the time from organ procurement to transplant, hearts harvested from donors after cardiac death are exposed to much greater ischemic insult events, including a hypoxemic arrest event, warm ischemic injury occurring during the mandatory five-minute stand-off period before organ harvesting may be commenced, and further ischemic injury occurring during reperfusion of the heart after it is harvested. Because of the ischemic damage that occurs before organ harvesting commences, hearts from non-heart-beating donors are not used for transplantation.